For example, DNV allows welding procedures to be approved in one of three different ways. The ship classification societies all require approved welding procedures to be used.

Preheat and inter-pass temperatures, and postweld heat treatment (if required).

Joint design, welding sequence and welding position.

Parent materials type, grade and thickness range.

Sometimes lists of approved welding consumables are issued by the Classification Society (such as ABS, Lloyds, DNV and RINA).

Welding consumables (wire, rods and gases).

Welding travel speed and/or heat input.

Voltage and current ranges, and polarity.

For arc welding these 'essential variables' include It specifies the welding parameters that must be adhered to for compliance to that welding procedure. The weld procedure specification (or WPS) is the 'recipe' for carrying out a particular weld. 3 Getting weld quality right 3.1 Approved welding procedures The sharp profile of these flaws makes them strong stress concentrators, and it is these flaws that most weld quality codes and standards are seeking to avoid.

In terms of structural integrity, the most critical flaws are cracks, lack of fusion or penetration, and undercut. The least severe are gas and slag inclusions, and weld shape flaws. Typical welding flaws can be classified into three categories to assess their significance planar flaws (cracks, lack of fusion or penetration, undercut or overlap) volumetric flaws (porosity and slag inclusions) and weld shape flaws (misalignment, or incorrect profile). Despite it being a major subject in itself, distortion is associated with even the best welds and is not a weld quality issue as such, so will only be touched on briefly in this paper. Most distortion is corrected after welding using localized flame spot heating to restore the required dimensions.Дistortion associated with welds may cause problems for the ship design strength and stiffness, and for the appearance of the finished vessel, and preventing and remedying distortion can be a major cost of shipbuilding. Using jigs and fixtures or pre-setting the components to offset the eventual distortion can also help. However, distortion can be minimised in welds that use low heat input and avoid excessive weld bead sizes. Any process that uses a localised heat source, such as welding, is likely to result in some distortion. Shipbuilders may consider the main problem with welding is the resultant distortion that occurs.

2 The problems with welding 2.1 Distortion The approach is similar to that for many safety critical construction applications - using qualified welders and welding procedures. Rules for shipbuilding are all written with the expectation of achieving safe shipping, including Lloyds Register, Det Norske Veritas, the American Bureau of Shipping, and also the International Association of Classification Societies, IACS. Ensuring good quality welding forms an important part of all ship classification societies' rules. Over the past sixty years of developments in welding technology, the current expectation of weld quality has never been higher. When welds fail, often the whole structure fails. Welding is one of the most critical operations within ship construction. Methods such as fitness-for-service assessment and fatigue improvement techniques can be applied to justify whether imperfect welds might still be adequate. However, it may not always be necessary to cut out the weld and start again. For shipbuilding, weld repair is usually the main strategy, and the importance of repair welding will be discussed. There are also a number of methods that can help to remedy the effects of poorer quality welding. However, no matter how rigorous the quality control, sometimes things go wrong. This paper summarises how consistent weld quality is achieved in practise, through classification society rules, welding procedure qualification, welder certification and weld monitoring and control.

The expectation of weld quality has never been higher. Fortunately, over sixty years of research and development in the field of welding has provided current ship builders with fabrication processes that are readily automated, can produce consistent welds reliably, and/or can weld thick sections in a single pass for controlled distortion. Paper presented at Analysis and design of Marine Structures 2nd International Conference on Marine Structures (MARSTRUCT 2009) March 16- 18, 2009 Lisbon, Portugal.

National Structural Integrity Research Centre.

Structural Integrity Research Foundation.

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